SPID#: 21 The chimpanzee is the only nonhuman primate that can be reproducibly infected with HIV-1. Typically, infection with HIV-1 in the chimpanzee does not lead to disease. As such these animals have mainly been used as a system for testing vaccines designed to prevent infection. The Yerkes Center houses 10 HIV-1-infected chimpanzees, most of which have been infected for over 10 years. These animals are monitored on a yearly basis for virus isolation and antibody responses. One of these animals, C499, developed a chronic diarrhea in March, 1995, which lasted throughout the year and was not associated with identifiable pathogenic microbial organisms. Treatment was unable to resolve the condition. In September 1995, C499 developed an acute watery diarrhea which was associated with the presence of large numbers of two organisms, Balantidium coli, and Blastocystis hominis. Treatment with anti-microbial and anti-protozoal drugs was able to resolve this condition. At this time, it was noted that the CD4+ cell level of C499 had become extremely low (minimum 10 cells/ml). HIV-1 was easily isolated from the PBMC of this animal, and as such, the diagnosis of AIDS was made. This represented the first development of AIDS in an HIV-1-infected chimpanzee. Virus isolated from C499 was able to replicate to high titers and induce syncytium formation in normal chimpanzee PBMC-- characteristics that have only been documented for two other HIV-1 isolates. An analysis of viral loads in the plasma of C499 showed that, using the Chiron b-DNA assay, RNA was undetectable prior to 1993, when CD4+ cell levels began decreasing. These studies suggest that AIDS can occur in chimpanzees infected with HIV-1, and that adaptation of the virus to the chimpanzee is associated with the development of pathogenic characteristics for chimpanzee cells in vitro.